In recent years, there have been few researches and analysis published on the seismic activity and stress state in Shandong segment of Tanlu fault zone using digital seismological methods such as seismic apparent stress, focal mechanism solution and so on. In this paper, source parameters such as focal mechanism solutions and apparent stress are calculated using the waveform data of M_L≥1 moderate-small earthquakes in Shandong segment of Tanlu fault zone recorded by Shandong digital seismic network since 2007. According to focal mechanism solutions, a statistical analysis is done on the focal dislocation types in the study area using triangle graphical method, and the results show that the faulting in this area is mainly of strike-slip mechanism, and there are less thrust and normal mechanism. Calculation with the mean stress tensor method illustrates that the direction of mean principle stress of Shandong segment of Tanlu fault zone is NEE-SWW, which is the result of the combined effect of the subduction of West Pacific plate and the extrusion of Indian plate to Eurasian plate; the small dip angle indicates that the mode of action of stress is nearly horizontal, and the direction of principal stress axis is nearly perpendicular to the Tanlu fault zone. Under the action of such compressive stress field, dislocation is not likely to occur and the stress accumulation is enhanced on both sides of the fault. The apparent stress is calculated using the source spectral parameters method. Apparent stress has positive correlation with the magnitude and increases with the increased magnitude. So we get apparent stress difference by subtracting the empirical fitting value from the apparent stress. By removing the impact of magnitude, and according to the temporal-spatial evolution image of apparent stress difference, we found that the apparent stress in Shandong segment of Tanlu fault zone generally has a trend of decrease starting from the Wenchuan earthquake in 2008, and the spatial distribution of apparent stress in the region is very uneven. Combined with the spatial distribution of b values, the result shows that high stress is mainly located in Anqiu segment and Tancheng-Juxian segment, especially in Anqiu segment where small magnitude earthquakes appeared accompanying with the high stress. Low b-value means high stress and low frequency means low stress release, which indicates that Anqiu segment might accumulate higher stress and is at the fault locking stage. The research will provide new data for better understanding the present active feature and stress state of the Shandong segment of the Tanlu fault zone.

An M_S4.6 earthquake occurred at noon on Nov. 23, 2013 at Laizhou, Shandong Province, China. This earthquake is the largest event since the Sept. 20, 1995 Cangshan M_S5.2 earthquake in Shandong area, and shook the whole Shandong Peninsula. The local area has low seismic activities, only one M_L3 earthquake sequence was recorded from 1970 to 2012. But since 2012, small shocks break out every now and then, up to the recent M_S4.6 sequence.We investigate the faulting process of the 2012—2014 Laizhou M4.6 earthquake sequence by combining relocated hypocenters and focal mechanisms. CAP method and additional bootstrap technique are employed to stably invert the moment tensor solution and to estimate its uncertainties. The average faulting parameters are: A. strike=239.6°, dip=75.0°, rake=174.4°; B. strike=331.1°, dip=84.6°, rake=15.0°, and error range of P, T axes is about 20°。We use HASH method to solve the focal mechanism solutions for 12 small events(M_L≥3.0)in the sequence, and adopt double difference method(HypoDD)to analyze precisely the aftershock distribution.Relocation images show that, except 3 small shocks away from the swarm, the concentrated area of Laizhou sequence presents a NE-oriented major axis, and the sources distribution indicates a NW dipping fault, with a dip angle about 70°, which is in accord with the solutions for small events retrieved by HASH method.Finally, a discussion on the structural features of seismic tectonic and faulting process is made by using of all the results and relative geological data, and several opinions are concluded as follows:(1) There was an ordered rupture process at the earlier stage. At the very beginning(Jan 1, 2012 M_L 3.2), rupture spread towards northeast. After the M_S4.6 mainshock, rupture of the aftershocks became disordered, and sources distribution became more stochastic.(2) Small events before the mainshock scattered around the main rupture area; the occurrence of M_S4.6 event filled up the gap.(3) Strike-slipping is the dominant faulting type in the earlier stage of the sequence. Two foreshocks right before the mainshock display some thrust component. This maybe implicates the strengthening of regional stress relative to the mainshock. The focal mechanism variation of small aftershocks indicates stress field's adjustment at deep source area after the mainshock.(4) Slipping vectors of the fault are in accord with accurate location results, which reveals the dynamics of faulting process.(5) The seismotectonic characters of Laizhou earthquake sequence revealed by this paper are consistent with other regional geology data. Focal mechanisms conform to the orientation of regional maximum horizontal principal compressive stress. This implies that Laizhou earthquake sequence occurred under the regional stress field, and has relationship with the relative motion between tectonic blocks.

Based on the differences of seismic activity,focal mechanism,geological structure,and hydrologic condition between reservoir induced seismicity(RIS)to natural earthquake,the basic features of reservoir induced earthquake from natural earthquake are studied statistically by the RIS database of 150 cases of reservoir induced earthquakes world-wide,and the results are shown as follows: ① Using statistical principle,we find that RIS occur in the areas near the reservoir banks in the impounding period,most of reservoir induced earthquake are distributed within a distance of 10km to the bank,with depth less than 10km,and a few events extend outwards as far as over 10km along fault zones,karst caves,and(thermal)springs; ②The frequency and magnitude of reservoir induced seismicity change with the reservoir water level. Most maximum magnitude earthquakes take place in the period before the first highest water level is reached,or in the following 2-3 impounding stages after the first highest water level,after then,a few would take place; ③The sequence of RIS is complete,usually as "foreshock-main shock-aftershock" or "foreshock-swarm-aftershock",most of them are microseisms of M_L1-3, so the b value is larger than natural earthquakes; ④ RIS differ from natural shocks in higher epicenter intensity,higher frequency of ground motion and higher peak ground acceleration,but faster attenuation with distance; ⑤The stress drop of RIS is smaller than that of natural earthquake of the same size,and the source size is larger; ⑥ The RIS occur in areas under lithological and geologic conditions of good permeability. Tensional fracture,tensional fault zone,unconsolidated fault plane,karst,and(thermal)springs are all good water penetration channels.

In this paper,a statistical analysis is made using the gray clustering method on the earthquake inducing factors,such as dam height,storage capacity,regional stress state,fault activity,lithology medium conditions and seismic activity background,based on a database of reservoir-induced earthquakes,which includes 150 reservoir-induced earthquake cases and 532 large reservoirs data. Corresponding earthquake occurrence ratios are obtained and used as predictors for reservoir-induced earthquakes. According to the six quake-inducing factors,assessment is made on the maximum magnitude of reservoir-induced earthquake for the Three Gorges Reservoir dam area,the bayou of Xiangxi stream and the cross-river section of the Xiannushan Fault zone,the west Xietan,Badong,and on both banks of Shenlong stream. The results show that the upper-limit of magnitude at dam area is less than magnitude 3,that at the bayou of Xiangxi stream and the cross-river section of Xiannushan Fault zone is between magnitude 4～5,that at west of Xietan is less than magnitude 3,and that at the northern bank of badong and on both banks of the Shenlong stream is between magnitude 3～4.The results have been validated by the actual reservoir-induced seismicity after the impoundment of the Three Gorges Reservoir, with the M_L 2 earthquake at Xiaoxikou 4 km far away from the dam of the Three Gorges Reservoir on February 6, 2009, the M_S 4.1 earthquake at the bayou of Xiangxi stream and the cross-river section of the Xiannushan Fault zone on November 22,2008, the M_S 2.6 earthquake at the west Xietan on August 22, 2009, and the M_L 3.5 earthquake at the bank of Shenlong stream on December 14,2009.

Using the observations of the Zipingpu reservoir seismic network(including seven short-period seismic stations,with an average station distance of 10km)of the period from July 2004 to 2009,and the data from January-December 2009 recorded by the intensive seismic observation network(six short-period seismic stations with the station spacing of 3km),930 foreshocks near Zipingpu reservoir,the main shock,and 5789aftershocks,as well as the initial rupture of the M_S 8.0 Wenchuan earthquake were relocated with double difference location algorithm.Combined with the geological field investigations,the analysis of seismic activity in Zipingpu reservoir area,the distribution of coseismic surface rupture,the depth of initial rupture location of Wenchuan M_S 8.0 earthquake,and the aftershock distribution,we find that the occurrence of M_S 8.0 Wenchuan earthquake has close relation with Zipingpu reservoir: 1) The seismic swarms at the Shuimo,and Dujiangyan and Shenxigou area are 10 kilometers away from the reservoir,the seismic strain release caused by Zipingpu reservoir increased by 200%,and the release was closely related to water level changes and accelerated before the Wenchuan earthquake; 2) There are two northeast directed coseismic surface rupture zones with 1m displacement along Zhongtanpu Fault; 3) the aftershocks of Wenchuan earthquakes are mainly distributed on northwestern wall of the Zhongtanpu Fault; 4) The relocation revealed that the Wenchuan earthquake occurred at 27minutes,59.5 seconds,the depth is between 6～9km; and 5) by comparison,the focal depths of the fore-shocks occurring on 2008-04-05 are about 7.8km,and the initial time of waveforms of this swarm is same as that of the M_S 8.0 Wenchuan earthquake.

815 aftershocks of Wenchuan earthquake recorded by 12 seismic stations in 2009 were relocated using the double difference algorithm to analyze the seismic activity of the Zipingpu reservoir.Relocation results show that the epicenters of aftershocks concentrate relatively in three regions.Focal depth distribution characters of different concentration regions take on obvious difference.This means earthquakes in different concentration regions may have different causes.Compared with relocations of earthquakes taking place before the Wenchuan earthquake done by other researchers,locations of the seismic concentration regions in the reservoir area changed noticeably after the Wenchuan earthquake.These variations are related with the stress adjustment of local part in the reservoir area and may also be related with the diffusion depth and range of increased pore pressure caused by rock failure in the course of Wenchuan earthquake.

136 earthquakes,taking place in the west of Xietan area,recorded by portable stations deployed in the Three Georges reservoir area were relocated using the double difference algorithm.The relocations show that the root-mean-square shifts of the relocations in the directions of E-W,N-S and U-D are 0.38km,0.33km and 0.98km,respectively.The earthquakes in clasolite area with the focal depths about 4～5km take on linear distributions from shallow to deep part.These earthquakes were deduced as reservoir induced earthquake of fault fracture type.In the contrast,the earthquakes in limestone pavement with the focal depths about 2～3km take on slightly divergent distribution and have the characters of reservoir induced earthquake of the karts collapse type.

In accordance with the requirements of "Eleventh Five-Year" national scientific and technological support projects,an intensive seismic observation network consisting of 26 seismic stations was established at the Three Gorges Reservoir area in the section of Hubei Province in March 2009(21 short-period seismographs,5 broadband digital seismographs).From March to December,2009,2995 M_L-0.8～2.9 earthquakes were observed during the trial impounding of the Three Gorges Reservoir(water level rose from 145m to 172.8m).Using the double difference earthquake location algorithm,2837 earthquakes were relocated precisely.The results show: The pattern of small local earthquake swarms in Three Gorges Reservoir area took on a linear distribution or mass-like cluster distribution,the mass-like clusters of events were generally within the distance of 5km from waterfront,and the linear distribution of the earthquakes could be extended to a distance of 16 kilometers away from waterfront.In Hubei section of Three Gorges Reservoir,earthquakes were mainly concentrated in the northern end of Xiannüfeng and Jiuwanxi Fault in the vicinity of the Xiangxihe river,and along the banks of the Yangtze river at the east of Xietan village and at Shenlongxi area on the northern bank in Badong region;the focal depths were less than 10km,and 4km in average.Earthquake frequency of the reservoir region had a positive correlation with reservoir water level fluctuation,indicating that the seismicity belonged to the reservoir induced earthquakes.Along the Shenlong river in the reservoir area,earthquakes showed three linear distributions in the northern Badong County,which was related to karst distribution.There are underground rivers along carbonate strata.When reservoir was impounded,water permeated into the underground rivers,thus inducing earthquakes.Earthquakes in the areas on the cross-river segment of Xiannüfeng Fault,the Jiuwanxi Fault and at the areas west of Xietan,Shazhen and Xizhen,may be related to the softening of discontinuities,such as the Nukou Fault,the Xiannüfeng Fault or the bedding joints,etc.,which would lead to failure of rock masses,thus,inducing earthquakes.However,convincing conclusions about the triggering mechanism still need further study.In addition,in the areas south of Wenhua and Yanglin of Zigui County and at Rangkou town east of Badong county,there are mining-induced earthquakes at the mines nearby.And on the shores of the reservoir,there are some collapse earthquakes.

In the paper,according to the related factors and characteristics of identified reservoir induced earthquakes,the parameters of reservoir induced earthquake database and library structure have been determined.Based on the database functions of ARCGIG geographic information system software,the reservoir induced earthquake database contains 131 reservoir induced earthquake cases in the would, 110 Class-Ⅰlarge reservoirs(larger than one thousand million cubic meters),200 ClassⅡlarge reservoirs (larger than one hundred million cubic meters and less than one thousand million cubic meters) and 70 reservoirs with dams higher than 100 meters in China.It has the functions of quick query,statistics and icons.After the Wenchuan earthquake,aftershocks endangered many reservoir's dams and people who were taking part in earthquake rescue and relief operations.Two days after the M_s8.0 Wenchuan earthquake,the distribution and basic information of large reservoirs in Sichuan Province and nearby regions was submitted.The government made timely earthquake emergency response decisions to ensure a safe relief.The reservoir induced earthquake database is not only a method provided to the government for quick decision-making,but also can be used nation-wide.

In this paper,the theoretical foundation of classification of focal mechanism solutions is drawn by the theory of shear dislocation,where the mechanisms of earthquake can be divided into the strike-slip fault,reverse fault and normal fault.The 124 focal mechanisms from Harvard University have been classified by the triangle diagram method in the area of Sichuan and Yunnan Provinces in China.The focal mechanisms of strike-slip,reverse and normal faulting have been discussed.These are the three deformation types in this area.The results show a SE-directed block movement of the Sichuan-Yunnan region.Along the Xianshuihe Fault,the Anninhe Fault,the Zemuhe Fault and the Xiaojiang Fault,large strike-slip movement and deformation have occurred.On the Sagaing Fault and the Longmenshan Fault,the reverse faulting mechanism dominates.The quick movement of the Red River Fault towards southeast driven by the Xiaojiang Fault forms a large tensional stress area between the Jinshajiang and the Xiaojinhe Fault.This constitutes the latest pattern of recent crustal deformation of Sichuan-Yunnan region and its adjacency.